One scenario the Windows RSS platform does not solve is accessing your feeds from anywhere. It allows you to subscribe to feeds on one Windows machine, and lets all applications on that machine access this feed store. However, if you are on another machine, you cannot access it.
I am using NewsGator to access my feeds from anywhere. I subscribed for free to a NewsGator online account, imported my OPML without problems, and can access my feeds now at home, at work, in an internet cafe.
I have a nice feed list in the browser, which just displays unread items, latest on the top, and I can open up the linked web pages in new tabs with IE7. If there are many news items, they put them in several pages, and you can mark all items on a page as read with one click. The read/unread status is also persisted online, so if you go to another machine, you will always see only the new items.
They even have a free news reader for Windows XP Media Center Edition. I don't like to read news items on the TV, but I created a second NewsGator online account and just subscribed to the Channel 9 video feed. I can watch the Channel 9 videos on my TV that way.
NewsGator has announced plans to integrate with the Windows RSS platform. Microsoft has similar ideas with Windows Live, but Niall Kennedy now announced he is leaving.
I still think the Windows RSS platform is a good idea, but for now I am reading my feeds with NewsGator.